Department Colloquium: Understanding how sunlight powers the world - The role of x-ray science

I will try to present my subjective view on how x-ray science can help addressing the challenge of making available new sources of energy. Starting from a fundamental understanding of photocatalytic and photochemical processes, our aim is to ultimately use this fundamental knowledge to help finding new concepts for the efficient transformation of photon energy into chemical energy. Key to this is to probe and characterize transient intermediates in photochemical reactions and of particular interest are metal complexes where molecules are bound to metal atoms. These are essential in various processes from photocatalytic solar-fuel production to photosynthetic water splitting with metalloproteins.

Large-scale x-ray sources in general and synchrotron radiation sources and x-ray free-electron lasers in particular with tuneable, intense and short x-ray pulses now enable a completely new class of experiments in this direction. Probing the essential properties locally at the metal site and as the system functions allows for unprecedented mechanistic insight at the molecular level.

I will present our approaches to understanding the activity of transition metals in chemistry and biology. We start from photochemical reactions of simple metal complexes and use time-resolved x-ray spectroscopy to understand how bonds and molecules can be activated. Complementary x-ray spectroscopy experiments on bond-activating metalloproteins are used to understand the fundamental chemistry. The necessity of applying short x-ray pulses with durations down to the femtosecond level for these experiments will be highlighted. I will discuss how we extract information on chemical properties based on the experimental observables and emphasize the importance of effective and efficient theoretical approaches.

Probing weird bonding configurations with x-ray laser spectroscopy means understanding how bonds rearrange as the system functions and could build a basis for the rational design of photocatalysts.